for COCs treatment. Due to the electronegative character of chlorine substituents,
COCs can effectively be transformed via reductive pathways. Moreover, reductive
dechlorination has shown higher efficiency on highly chlorinated compounds.
This chapter focuses on the presentation of the chemical reduction of the most
common COCs pollutants, followed by kinetic and mechanistic approaches related
to the use of iron-based particles. Developments of in situ chemical reduction
technologies aiming to enhance remediation rates are also exposed. Influence of
environmental conditions for in situ applications is then developed. Finally, a case
study is presented.
Keywords Chlorinated organic compounds · Chemical reduction · Zero-valent
iron · Kinetics · Degradation pathways
6.1 Introduction
Chlorinated organic compounds (COCs) are common contaminants of soils and
groundwater. Given their main physico-chemical properties, they tend to accumulate
in the subsurface and persist from decades to centuries. Some COCs like
chloromethane and chloroform are naturally produced by living organisms, especially in marine environment—algae, sponges, fungi, and bacteria (Ballschmiter
2003; Gribble 2003)—but the main source is anthropogenic. For example,
chloromethanes production was about 2.785 million tons in 2014 (Sherry 2015).
Most chlorinated solvent families, like chlorinated methanes, chlorinated ethanes,
chlorinated ethenes, and chlorinated benzenes, were industrially produced by chlorination—addition, electrophilic or radical reactions—of saturated or unsaturated
hydrocarbons.
COCs such as perchloroethylene (PCE) and trichloroethylene (TCE) are used as
dry-cleaning solvents and metal degreasing agents as they are non-flammable, or as
chemical intermediate for various applications—for example, trichloromethane is
used as an intermediate in the production of organic fine chemicals—or extracting
agents, or as functional fluids, e.g., hydraulic fluids in industrial equipment
(European Chlorinated Solvent Association). Some of them, such as
hexachlorobenzene (HCB), have been used as pesticides (Hawley 1981;
Verschueren 1983; Budavari 1996). Their uses have been regulated, and production
tends to be well reduced. For example, PCE production in United States dropped
from 318 million tons in 1970 to 45 million tons in 1994 (Doherty 2000a, b).
In the environment, COCs are often transported into the subsurface by vaporphase migration, by infiltration of contaminated water, and as a moving dense
nonaqueous phase liquid (DNAPL) (Johnson and Pankow 1992). The life cycle of
a COCs source zone can be resumed as: (1) DNAPL release, (2) DNAPL redistribution, (3) dissolution and aging, (4) depletion, and (5) back diffusion and desorption (Kueper et al. 2014). The relatively low viscosities of COCs (same order of
magnitude as water) allow the rapid downward movement in the subsurface
284
R. Rodrigues et al.
COCs can effectively be transformed via reductive pathways. Moreover, reductive
dechlorination has shown higher efficiency on highly chlorinated compounds.
This chapter focuses on the presentation of the chemical reduction of the most
common COCs pollutants, followed by kinetic and mechanistic approaches related
to the use of iron-based particles. Developments of in situ chemical reduction
technologies aiming to enhance remediation rates are also exposed. Influence of
environmental conditions for in situ applications is then developed. Finally, a case
study is presented.
Keywords Chlorinated organic compounds · Chemical reduction · Zero-valent
iron · Kinetics · Degradation pathways
6.1 Introduction
Chlorinated organic compounds (COCs) are common contaminants of soils and
groundwater. Given their main physico-chemical properties, they tend to accumulate
in the subsurface and persist from decades to centuries. Some COCs like
chloromethane and chloroform are naturally produced by living organisms, especially in marine environment—algae, sponges, fungi, and bacteria (Ballschmiter
2003; Gribble 2003)—but the main source is anthropogenic. For example,
chloromethanes production was about 2.785 million tons in 2014 (Sherry 2015).
Most chlorinated solvent families, like chlorinated methanes, chlorinated ethanes,
chlorinated ethenes, and chlorinated benzenes, were industrially produced by chlorination—addition, electrophilic or radical reactions—of saturated or unsaturated
hydrocarbons.
COCs such as perchloroethylene (PCE) and trichloroethylene (TCE) are used as
dry-cleaning solvents and metal degreasing agents as they are non-flammable, or as
chemical intermediate for various applications—for example, trichloromethane is
used as an intermediate in the production of organic fine chemicals—or extracting
agents, or as functional fluids, e.g., hydraulic fluids in industrial equipment
(European Chlorinated Solvent Association). Some of them, such as
hexachlorobenzene (HCB), have been used as pesticides (Hawley 1981;
Verschueren 1983; Budavari 1996). Their uses have been regulated, and production
tends to be well reduced. For example, PCE production in United States dropped
from 318 million tons in 1970 to 45 million tons in 1994 (Doherty 2000a, b).
In the environment, COCs are often transported into the subsurface by vaporphase migration, by infiltration of contaminated water, and as a moving dense
nonaqueous phase liquid (DNAPL) (Johnson and Pankow 1992). The life cycle of
a COCs source zone can be resumed as: (1) DNAPL release, (2) DNAPL redistribution, (3) dissolution and aging, (4) depletion, and (5) back diffusion and desorption (Kueper et al. 2014). The relatively low viscosities of COCs (same order of
magnitude as water) allow the rapid downward movement in the subsurface
284
R. Rodrigues et al.